Electromagnetic ultrasound probe

ABSTRACT

Disclosed is an electromagnetic ultrasonic probe for coupling-media-free generation and reception of ultrasonic waves in the form of linearly polarized transverse waves in a workpiece ( 5 ), respectively from a workpiece ( 5 ), having a unit which generates the ultrasonic waves inside the workpiece ( 5 ) and which is provided with a transmission coil arrangement ( 7 ), to which a high-frequency voltage can be applied to generate a high-frequency magnetic field, and a premagnetizing unit (V) to generate a quasi-static magnetic field superimposing the high-frequency magnetic field in the workpiece ( 5 ), and an ultrasonic waves reception unit providing a reception coil arrangement ( 8 ) which can be connected to an evaluation unit, with the transmission coil arrangement ( 7 ) and the reception coil arrangement ( 8 ) being disposed torus-shaped at least on one partially toroidally designed magnetic core ( 6 ), which is provided with two front ends ( 11 ) which can be turned to face the workpiece ( 5 ) and via which the high-frequency magnetic fields can be coupled into, respectively coupled out of, the workpiece ( 5 ).  
     The present invention is distinguished by the premagnetizing unit (V) being contactable directly or indirectly with the workpiece ( 5 ) via a contact area ( 9 ) and by the at least one partially torodially designed magnetic core ( 6 ) being disposed laterally next to the contact area ( 9 ) of the premagnetizing unit (V) in such a manner that the premagnetizing unit (V) can project over the partially toroidally designed magnetic core ( 6 ) perpendicular to the contact area ( 9 ).

BACKGROUND OF THE INVENTION

The present invention relates to an electromagnetic ultrasonic probe forcoupling media-free generation and reception of ultrasonic waves in theform of linearly polarisized transverse waves in a workpiece,respectively therefrom. Such a type ultrasonic probe provides a unitwhich generates ultrasonic waves inside the workpiece having atransmission coil arrangement to which a high-frequency voltage can beapplied to generate a high-frequency field: moreover a premagnetizingunit ensures generation of a quasi-static magnetic field superimposingthe high-frequency magnetic field in the workpiece. Furthermore, todetect ultrasonic waves, a unit is provided to receive the ultrasonicwaves. The ultrasonic reception unit is provided with a reception coilarrangement which can be connected to an evaluation unit.

In order to prevent the filigree transmission coil and reception coilarrangements from suffering mechanical damage due to direct contact withthe surface of the workpiece, the arrangements are disposed in atorus-shaped manner on at least one partially toroidally designedmagnetic core, which is provided with at least two front ends that canbe turned to face the workpiece. The high-frequency magnetic fields canbe coupled into, respectively out of, the workpiece via these two frontends, permitting in this manner to dispose the coil arrangements on thesurface of the workpiece at a distance from each other. Nonetheless thehigh-frequency magnetic fields required for the generation and detectionof ultrasonic waves inside the workpiece are effectively coupled into,respectively out of, the workpiece via a partially toroidally designedmagnetic core.

Such type ultrasonic probes permit generation and reception of linearlypolarized transverse waves which are irradiated under the probeperpendicular into the workpiece, respectively are received from thisdirection and oscillate perpendicular to their propagation direction ina plane. Technical fields of application of such type ultrasonic probesare, for example, nondestructive examination of electrically conductiveworkpieces for material flaws, such as cracks, in particularperpendicular to the polarization direction of the ultrasonic waves andcrack-like flaws that are oriented parallel to the propagationdirection, including other process based on ultrasonic velocity andpolarization, such as for example measuring voltage or, in particular,measuring thickness.

PRIOR ART

The coupling-media-free electromagnetic probes known from the state ofthe art convert electromagnetic field energies in the elastic energy ofan ultrasonic wave and inversely. The conversion mechanism is based onthe interaction between the electromagnetic field and an electricallyconducting material that moreover a static magnetic field or aquasi-static magnetic field applied from the outside passes through. Theterm “quasi-static” magnetic field comprises, in addition to the actualstatic magnetic field, which for example can be generated by means ofpermanent magnets, also low-frequency magnetic fields, whose alternatingfrequency is much lower than the high frequency with which thetransmission coil arrangement is operated to generate high-frequencyfields.

In order to excite ultrasonic waves inside an electrically conductingworkpiece, at least one part of the high-frequency magnetic field, whosefrequency range lies within the ultrasonic frequency range, and which isgenerated by the high-frequency coil arrangement, is coupled into theworkpiece, thus inducing eddy currents at skin depth which inconjunction with the “quasi-static” magnetic field generate ultrasonicwaves due to the Lorentz forces or magnetostrictions occurring insidethe workpiece.

Detection of ultrasonic waves occurring inside the workpiece occursinversely by detection of the electric voltage induced in the receptioncoil arrangement resulting from high-frequency fields which for theirpart are generated by the motions of electric charges, due to theultrasonic waves, in the workpiece inside the “quasi-static” magneticfield.

All prior art electromagnetic ultrasonic transducers are based on thecommon goal of development to optimize measuring sensitivity and,related thereto, to optimize the signal amplitudes that can be producedby the transmission coil and the reception coil arrangements in thetransmission signal and in the reception signal. The aim, on the onehand, is to design the coupling mechanism with which the generated andto-be-detected high-frequency fields are coupled in and out between theultrasonic transducer and the workpiece as loss-free as possible and, onthe other hand, to select the field strength of the quasi-staticmagnetic field as large as possible, which is decisive for generatingand detecting ultrasonic waves.

DE 42 23 470 C2 describes a generic electromagnetic probe for verticalacoustic irradiation of linearly polarized transverse waves, in whichthe high-frequency magnetic fields are coupled in and out in a mostefficient manner between the probe and the workpiece without, as is thecase with many other probes, placing the transmission and receptioncoils, usually designed as high-frequency air coils, directly on thesurface of the workpiece. But rather the electromagnetic probe of FIG. 2described in this printed publication is provided with a half-opentoroidal tape core 6, made commercially of amorphous tape material,around which a transmission coil arrangement 7 and a reception coilarrangement 8, respectively, are wound. The front ends 11 of thehalf-open toroidal tape core 6 act as coupling areas for thehigh-frequency magnetic fields and can be placed in a suited manner onthe surface of the to-be-examined workpiece 5. The high-frequencymagnetic fields generated by the high-frequency transmission coilarrangement 7 reach via the front ends 11 of the toroidal tape core 6into the workpiece 5 and are able to induce close-to-the surface eddycurrents 12 at skin depth inside the workpiece 5.

The quasi-static magnetic field oriented perpendicular to the surface ofthe workpiece 5 required for sound conversion is generated by means oftwo permanent magnets 3 of the same name and is conveyed to the materialsurface of the workpiece 5 via a soft iron core 2 provided inside thetoroidal tape core. The premagnetizing unit required for producing the“quasi-static” magnetic field that is oriented perpendicular to thesurface of the workpiece is located inside the open part of the toroidaltape core 1.

A drawback of the aforedescribed embodiment of an electromagnetic probeare the low signal strengths obtainable with this probe for generatingand detecting ultrasonic waves. Thus the construction-based low volumeof the premagnetizing unit prevents generating high magnetic fluxes.

In addition to the preceding printed publication, DE 36 37 366 A1 and DE195 49 207 A1 describe probes for nondestructive examination. However,the construction of these probes differs from that of the ones describedin the preceding. For instance, DE 36 37 366 A1 presents anelectromagnetic ultrasonic transducer, whose high-frequency transmissionand reception coil arrangement are placed along a rib-like carrierstructure over which a magnetic arrangement for generating thequasi-static magnetic field projects. DE 195 49 207 A1 describes acorresponding probe which is provided with a magnetic fieldconcentrating element but differs in all other details from the devicedescribed in the introduction.

SUMMARY OF THE INVENTION

The object of the present invention is to further develop a genericelectromagnetic ultrasonic probe in such a manner that the linearlypolarized transverse waves that can be generated inside a workpiece withthe probe have higher signal amplitudes than is the case with hithertoprobes. In particular, detection sensitivity of such a type probe shouldbe increased without having to place the high-frequency coilarrangements required for generating and receiving the ultrasonic wavesnear to the surface of the workpiece.

The solution of the object of the present invention is set forth inclaim 1. Advantageous features that further develop the inventive ideaare put forth in the subordinate claims.

The present invention is based on the idea of providing a selectivepossible manner of increasing the construction volume of thepremagnetizing unit required for the “quasi-static magnetic field” basedon the electromagnetic probe of DE 42 23 470 C2. Enlarging thepremagnetizing unit permits increasing the strength of the“quasi-static” magnetic field entering the workpiece perpendicularthereto in such a manner that the Lorentz forces or themagnetostrictions responsible for generating ultrasonic waves inside theworkpiece are increased, which ultimately leads to ultrasonic waves ofgreater amplitude.

Inversely, the construction-based magnetic field enhancement of the“quasi-static” magnetic field entering the workpiece results in theformation of stronger high-frequency fields which are generated byultrasonic-wave-based charge excursions inside the workpieces in thepresence of quasi-static magnetic fields and are coupled into thetoroidal tape core via the front ends. These high-frequency fieldsenable inducing higher electrical voltages into the reception coilarrangement, by means of which the detection sensitivity of theelectromagnetic ultrasonic transducer can be improved considerably.

A key element of the invention is that an electromagnetic ultrasonicprobe according to the generic part of claim 1 provides a premagnetizingunit which can be contacted directly or indirectly with the workpiecevia a contact area. The premagnetizing unit is disposed laterally besidethe at least one partially toroidally designed magnetic core, preferablydesigned in the form of a toroidal tape core, in such a manner that thepremagnetizing unit is able to project over the partially toroidallydesigned magnetic core in a manner perpendicular to the contact area.

Contrary to the aforecited DE 42 23 470 C2, the preferably half opentoroidal tape core, due to the arrangement, does not project over thepremagnetizing unit, but rather the toroidal tape core is locatedlaterally directly beside the premagnetizing unit without projectingover the premagnetizing unit or even just parts of it in projection tothe surface of the workpiece.

In a preferred embodiment, the partially toroidally designed magneticcore built as a toroidal tape core is disposed inclined with regard toits toroidal plane at an angle α to the contact area, with the two frontends of the toroidal tape core facing the workpiece also forming anangle α with the toroidal plane in such a manner that the toroidal tapecore lies largely flush via the front ends on the workpiece. The inclineof the toroidal plane of the toroidal tape core is preferably formed insuch a manner to the contact area that the high-frequency magneticfields that can be coupled into the workpiece via the front ends extendin the region under the contact area between the premagnetizing unit andthe workpiece and in this way interacts with the quasi-static magneticfield inside the workpiece to generate eddy currents.

The lateral arrangement of at least one partially toroidal magnetic corerelative to the contact area between the premagnetizing unit and theworkpiece permits constructing the premagnetizing unit, for example inthe form of one permanent magnet or a multiplicity of permanent magnets,as large as desired, in particular in the vertical extension to thecontact area in order to generate a desired strong “quasi static”magnetic field. Dimensioning of the premagnetizing unit is unlimited dueto the fact that the ultrasonic transducer arrangement is open upwardperpendicular to the contact area. Only handling aspects can limit thesize.

Based on the arrangement of an electromagnetic ultrasonic transducerknown from DE 42 23 470 C2, a simple embodiment provides for the use ofa single toroidal tape core along which both a high-frequency coilarrangement for a transmission unit and a reception unit are wound.

One preferred embodiment, however, provides for two partially toroidallydesigned magnetic core constructed as toroidal tape cores disposed onopposite sides with regard to the contact area and thus relative to thepremagnetizing unit. Due to separate signal conducting and in order toprevent mutual interference of the transmission signals and thereception signals, the transmission coil arrangement and the receptioncoil arrangement are disposed separate from each other on the toroidaltape cores located opposite the premagnetizing unit. Such a typeseparate coil arrangement on two separate toroidal tape cores primarilycontributes to reducing so-called dead times which occur if thetransmission coil arrangement and the reception coil arrangement aredisposed on one and the same toroidal tape core. Ultimately this leadsto the toroidal tape core being unable to detect reception signalsduring the short time periods in which the transmission coil arrangementconveys the toroidal tape core into saturation magnetization. If theultrasonic transducer is utilized, for example for measuring thickness,these saturation effects lead to the occurrence of dead times which areinaccessible for measuring thickness, i.e. due to the temporallyimmediate succession of transmission signals and reception signals, thereception signals can, due to the saturation effect be detected onlyfollowing a minimum interval after the transmission signal. Therefore,workpieces must have at least a thickness of 3-4 mm to be measured.

In a further preferred embodiment, the arrangement is provided with twopairs of toroidal tape cores disposed orthogonally in relation to eachother about the contact area, which preferably is rectangular in shape.In this manner, two linearly polarized transverse wave fields can begenerated inside a workpiece with oscillation planes that are orientedperpendicular to each other, respectively. In this manner, materialflaws, for example in the form of cracks the course of which areoriented either perpendicular to one or the other oscillation plane, aredetected precisely.

For an efficient as possible, i.e. low loss, and concentratedcoupling-in of the “quasi-static” magnetic field into the to-be-examinedworkpiece, a preferred embodiment provides for utilizing two same namepermanent magnets which are both at least partially enclosed by a blockmade of a soft magnetic material. Connected to the block in direction ofthe workpiece is a concentrator, which, for its part, also contains asoft magnetic material in order to concentrate the magnetic flux on thecontact area. The concentrator itself is provided with two differentsized surfaces opposite each other. The larger surface of the two isconnected to the soft magnetic material workpiece which at leastpartially encloses the permanent magnets and the smaller one faces theto-be-examined workpiece, which defines the contact area between thepremagnetizing unit and the material. As mentioned in the preceding, thecontact area is preferably rectangular in shape and along its at leasttwo opposite lateral edges are disposed the toroidal tape cores inclinedobliquely to the contact area.

In order to prevent disturbing eddy currents from coupling in due to thegenerated high-frequency fields inside the concentrator as a result ofthe proximity to the toroidal tape cores conducting the high-frequencyfields, the concentrator is not composed of a homogeneous electricallyconducting material but rather of an electrically nonconducting materialinto which ferromagnetic particles are introduced in a matrix-likemanner in order to conduct and to concentrate the magnetic flux or theconcentrator is constructed like the magnetic core of a transformer andcomprises a multiplicity of stacked metal plates.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is made more apparent in the following usingpreferred embodiments with reference to the accompanying drawingswithout the intention of limiting the scope or spirit of the overallinventive idea.

FIGS. 1 a, b show a lateral and a front view of an electromagneticultrasonic transducer according to the present invention and

FIG. 2 shows a state-of-the-art electromagnetic transducer.

WAYS TO CARRY OUT THE INVENTION, COMMERCIAL APPLICABILITY

FIGS. 1 a and b show a lateral and a front view of a preferredembodiment of an electromagnetic ultrasonic transducer designedaccording to the present invention comprising a premagnetizing unit Vfor generating a sufficiently strong static magnetic field 1 whichenters the to-be-examined material W in a perpendicular manner. Thepremagnetizing unit V is provided with two same name permanent magnets 3which are at least partially enclosed by a soft magnetic material 2, viawhich the magnetic flux H is introduced into the workpiece 5perpendicular to its surface via a concentrator 4, which is connected tothe material 2.

The front view of FIG. 1 shows that the concentrator 4 has a taperedshape tapering toward the contact area 9 between the premagnetizing unitV and the surface of the workpiece, due to which the magnetic flux Hconducted inside the concentrator 4 is concentrated on the narrowlyconfined contact area 9.

A half open toroidal tape core 6 is disposed to the left and to theright of the sides of the concentrator 4. The transmission coilarrangement 7 is placed on one of the toroidal tape cores 6 and thereception coil arrangement 8 on the other. The ring tape cores 6 areinclined toward the contact area 9 with regard to their partiallytoroidal planes 10 in such a manner that, on the one hand, it is ensuredthat the premagnetizing unit V can assume any desired constructionheight perpendicular to the contact area that projects over the toroidaltape core and, on the other hand, the coil arrangements 7,8 arepositioned at a distance from the workpiece surface, due to which theyare not subject to any mechanical wear as a result of direct contactwith the surface of the workpiece 5.

In order to ensure that the high-frequency magnetic fields can becoupled in from, respectively to, the individual high-frequency coilarrangements 7,8 largely with no losses into the workpiece 5 via thesurface of the workpiece, the front ends 11 of the the toroidal tapecores 6 form with the respective partially toroidal plane 10 an angle αas well, which can fundamentally be selected between 0° and 90°,preferably however is between 30° and 60°. In this manner it is ensuredthat, despite the inclined position of the toroidal tape cores 6, thefront ends 11 lie flush to the sides of the contact area 9 on thesurface of the workpiece 5, permitting coupling in and out of thehigh-frequency magnetic fields largely without losses.

To trigger the ultrasonic waves, the transmission coil arrangement 7,which is usually connected to a high-frequency generator, is fed ahigh-frequency current burst signal. The magnetic alternative flux H_(w)generated by the transmission coil arrangement 7 reaches into theworkpiece 5 via the toroidal tape core 6 and via a small air gapenclosed between the front ends 11 and the surface of the workpiece 5. Aspatially homogenous magnetic alternating field H_(w) forms at skindepth in the workpiece between the front ends 11 of the toroidal tapecore bearing the transmission coil arrangement 7. The eddy currentscoupled with the magnetic alternating field H_(w) inside the workpiece 5are superimposed by the magnetic field entering the workpiece 5perpendicularly via the concentrator 4, thereby generating, due to theforming Lorentz forces and magnetostricitions, ultrasonic wavesoscillating perpendicular to the direction of the eddy currents andpropagating perpendicular to the surface of the workpiece, respectivelyto the contact area 9. The arrow representations indicate theoscillation direction S and the propagation direction A.

The reception mechanism for detecting the ultrasonic waves propagatinginside the workpiece is based on the inverse effect, notably the soundparticle velocity developing inside the workpiece of the ultrasonic wavereturning to the probe, in interaction with the static magnetic field,generates an electrical field, which conducted via the toroidal tapecore 6 to the reception coil arrangement 8 induces an electrical voltagetherein. The electrical voltage induced in the reception coilarrangement 8 can usually be amplified with a downstream amplifier andcorrespondingly evaluated with an evaluation unit.

Thus the electromagnetic ultrasonic transducer designed according to thepresent invention combines the advantages relating to loss-free aspossible coupling-in and coupling-out of high-frequency magnetic fieldsrequired for generation, respectively detection, into, respectively outof, the to-be-examined workpiece. For this purpose, the coilarrangements required for generation and reception are disposed at adistance from the surface of the workpiece in an advantageous manner sothat they are not subject to any mechanical wear. Moreover, thearrangement designed according to the present invention offers almostany desired dimensioning of the premagnetizing unit in order to optimizethe magnetic field strength of the “quasi-static” magnetic field asdesired. This measure ultimately leads to generating greater signalamplitudes for producing stronger ultrasonic waves inside the workpiece,thereby permitting decisively improving the detection sensitivity of theelectromagnetic ultrasonic transducer. Dimensioning of thepremagnetizing unit is only limited by handling concerns.

LIST OF REFERENCES

-   1 “quasi-static” magnetic field-   2 soft magnetic material-   3 permanent magnets-   4 concentrator-   5 workpiece-   6 toroidal tape core-   7 transmission coil arrangement-   8 reception coil arrangement-   9 contact area-   10 partially toroidal plane-   11 front end-   12 eddy current-   S oscillation direction-   V premagnetizing unit-   A propagation direction

1-10. (canceled)
 11. An electromagnetic ultrasonic probe forcoupling-media-free generation and reception of ultrasonic waves in theform of linearly polarized transverse waves in a workpiece, respectivelyfrom a workpiece, includes a unit which generates the ultrasonic wavesinside the workpiece and which is provided with a transmission coilarrangement, to which a high-frequency voltage can be applied togenerate a high-frequency magnetic field, and a premagnetizing unit togenerate a quasi-static magnetic field superimposing the high-frequencymagnetic field in the workpiece; and an ultrasonic waves reception unitproviding a reception coil arrangement which can be connected to anevaluation unit, with the transmission coil arrangement and thereception coil arrangement being disposed torus-shaped at least on onepartially toroidally designed magnetic core, which is provided with twofront ends which can be turned to face the workpiece and via which thehigh-frequency magnetic fields can be coupled into, respectively coupledout of, the workpiece; and wherein the premagnetizing unit can becontacted directly or indirectly with the workpiece via a contact area,and the at least one partially torodially designed magnetic core isdisposed laterally next to the contact area of the premagnetizing unitin such a manner that the premagnetizing unit can project over thepartially toroidally designed magnetic core perpendicular to the contactarea.
 12. The electromagnetic ultrasonic probe according to claim 11,wherein the premagnetizing unit generates a quasi-static magnetic fieldwhose magnetic field lines pass through the contact area largelyperpendicular thereto.
 13. The electromagnetic ultrasonic probeaccording to claim 11, wherein the premagnetizing unit provides at leastone permanent magnets whose magnetic field lines can be concentrated bymeans of a concentrator on the contact area.
 14. The electromagneticultrasonic probe according to claim 13, wherein the at least onepermanent magnet is at least partly enclosed by a magnetic workpiecewhich bundles the magnetic field lines on the concentrator.
 15. Theelectromagnetic ultrasonic probe according to claim 13, wherein theconcentrator is made of a magnetic material and is provided with twosurfaces opposite each other, of which one is larger than the other andthe smaller surface determines the size of the contact area and thelarger surface is connected to the magnetic workpiece.
 16. Theelectromagnetic ultrasonic probe according to claim 14, wherein theconcentrator is made of a soft magnetic material and is provided withtwo surfaces opposite each other, of which one is larger than the otherand the smaller surface determines the size of the contact area and thelarger surface is connected to the soft magnetic workpiece.
 17. Theelectromagnetic ultrasonic probe according to claim 15, wherein theconcentrator is provided with an electrically nonconducting material inwhich ferromagnetic particles are embedded matrix-like, or theconcentrator comprises a stack-like arrangement of single metal plates.18. The electromagnetic ultrasonic probe according to claim 16, whereinthe concentrator is provided with an electrically nonconducting materialin which ferromagnetic particles are embedded matrix-like, or theconcentrator comprises a stack-like arrangement of single metal plates.19. The electromagnetic ultrasonic probe according to claim 11, whereinthe at least one partially toroidally designed magnetic core has apartially toroidal plane which forms with the contact area an angle awith 0°<a<90°, and the front ends of the partially toroidally designedmagnetic core form an angle a with the partially toroidal plane.
 20. Theelectromagnetic ultrasonic probe according to claim 11, wherein at leasttwo partially torodially designed magnetic cores are provided of whichone provides the transmission coil arrangement and the other thereception coil arrangement, and the partially toroidally designedmagnetic cores are disposed relative to the premagnetizing unit onopposite sides.
 21. The electromagnetic ultrasonic probe according toclaim 20, wherein the partially toroidally designed magnetic cores aredisposed axially symmetrically to a symmetrical axis passing through thepremagnetizing unit, and wherein the partially toroidal planes of thepartially toroidally designed magnetic cores each form an angle a withthe contact area.
 22. The electromagnetic ultrasonic probe according toclaim 11, wherein the at least one partially torodially designedmagnetic core is designed as a toroidal tape core.
 23. Theelectromagnetic ultrasonic probe according to claim 13, wherein thepremagnetizing unit provides two permanent magnets.
 24. Theelecetromagnetic ultrasonic probe according to claim 19, wherein30°<a<60°.